Efforts will continue to purify the putative natriuretic hormone (NH). Blood from volume expanded dogs will be subjected to affinity chromatography using a column constructed with anti-digoxin antibodies. This method will yield nanogram quantities of an endogenous digoxin-like substance (endoxin). This material will be further chromatographed by HPLC to ascertain purity. Once a pure preparation is obtained, amino acid composition will be determined by chromatography of an appropriate hydrolysate of the purified substance. The endoxin prepared by affinity chromatrography will be studied for its effects on renal function. The effects on renal electrolyte excretion, renal water excretion, renal blood flow and GFR will be monitored. Additional purification of a putative precursor of endoxin will be performed. Enzymatic digestion of this putative precursor will be carried out to determine whether endoxin can be produced as a breakdown product. Comparison of the breakdown product with the natural product to determine their identity will be performed.